This invention relates to a method and apparatus for rejecting nitrogen from a feed gas stream comprising methane and nitrogen so as to form a methane product.
It is known to extract natural gas from underground reservoirs. The natural gas often contains nitrogen. The nitrogen may be in part or totally derived from nitrogen which has been injected into the reservoir as part of an enhanced oil recovery (EOR) or enhanced gas recovery (EGR) operation. A feature of such operations is that the concentration of nitrogen in the natural gas tends to increase with the passage of time from about 5% by volume to about 60% by volume or higher.
U.S. Pat. No. 4,415,345 discloses a process for rejecting the nitrogen from the methane in a double rectification column operating at cryogenic temperatures. A double rectification column comprises a higher pressure rectification column, a lower pressure rectification column, and a condenser-reboiler placing the top of the higher pressure rectification column in indirect heat exchange with a region, usually the bottom, of the lower pressure rectification column. In the process according to U.S. Pat. No. 4,415,345 a stream of a mixture of nitrogen and methane is cooled at elevated pressure to a temperature suitable for its separation by rectification. A part of the feed gas is liquefied. The resulting gas mixture is separated by rectification. In one embodiment described in U.S. Pat. No. 4,415,345 a double rectification column is employed to carry out the separation. A liquid methane product is withdrawn from the bottom of the lower pressure rectification column and is raised in pressure by a pump. A waste nitrogen stream is withdrawn from the top of the lower pressure rectification column and is discharged from the plant.
The methane product is typically required at a similar pressure to that at which the natural gas is supplied, for example, typically in the order of 40 bar. With relatively high methane feed purity in the order of 95% it is possible to pump the liquid methane product to about 25 bar upstream of its vaporisation which is effected by indirect heat exchange with the incoming feed gas. The vaporised product methane may be raised further in pressure by compression.
As the mole fraction of methane in the feed gas decays and the mole fraction of nitrogen in it rises, so the feed gas becomes easier to separate. A designer of a separation plant faces the choice of whether to generate sufficient refrigeration so as to ensure that there is a high recovery of methane in the product stream throughout the operation of the plant, potentially at the cost of providing refrigeration circuits that are unnecessary at higher nitrogen mole fractions in the feed gas, or to exclude such circuits at the cost of a much lower methane recovery in the product stream at lower nitrogen mole fractions.